This invention relates to navigational aids, and in particular to a navigational aid for calculating the effect of a course and/or speed alteration of one sea-going vessel on the nearest approach distance of another such vessel.
In an encounter between two vessels which could result in a collision, the steering rules usually require one vessel to keep out of the way of the other. Only in the case of vessels meeting "end on" are both vessels required to take action, and alter course to starboard. The manoeuvre demanded in this latter case will always produce a clearing situation in which the bearing of one vessel from the other changes in an anticlockwise sense, i.e. each passes down the port side of the other. This anticlockwise rotation of the line of sight to the other vessel has become accepted by mariners as the usual convention for a safe passing, and in the ordinary practices of seamen the give way ship in other encounters usually tries to produce this condition. For example, a ship threatened by another ship closing from an angle on the starboard bow will invariably alter course to starboard, if sea room permits, and thus make the bearing change anticlockwise.
During periods of reduced visibility when ships are manoeuvring by radar information alone and both parties in the encounter are free to take action, their combined actions may accelerate rather than clear the collision risk if one attempts to make the bearing change clockwise while the other alters to make the change anticlockwise. In these situations safety may be increased if both ships alter in a complementary fashion (i.e. both act to rotate the sight line the same way). Keeping in mind the established practice of mariners which prefers to see an anticlockwise rotation of the bearing of the other ship, it follows that in these situations where ships can alter at random, action which will produce anticlockwise rotation is more likely to be successful since there is less chance of the other vessel taking an action which will cancel it.
Thus in all situations the preference of the mariner is to try to find an alteration which will produce anticlockwise rotation. A brief consideration of any situation when two vessels meet on a collision course, and then alter to avoid each other, must show that the effectiveness of the manoeuvre of your own ship is very dependent upon a number of factors. It will be obvious that it is never possible to forecast the eventual outcome of a situation if the intended action of the other ship must remain an unknown quantity, and thus the best a navigator can do is calculate, for a proposed alteration of his own course and/or speed together with the length of time for which the manoeuvre is to be held, how much his own ship will contribute to clearing the existing situation, and determine the direction in which the relative bearing of the ships will change.
In an encounter between two ships it is at present difficult for a navigator to determine the contribution to the avoidance of a collision which a given course and/or speed alteration of his own vessel will make towards avoiding a collision, the normal procedure involving a complex triangulation computation which requires that the true course and speed of the other vessel be determined. This not only wastes valuable time but the errors which may result from the initial computation of the true course and speed of the other vessel, and from the tracking which precedes it, can have a considerable effect on the effectiveness of the manoeuvre finally decided upon. Most ships are now fitted with radar apparatus the display of which enables the relative bearing of another ship approaching on a near miss or collision course to be determined quickly and accurately, and the present invention stems from the appreciation that it is possible to calculate the effect of a course and/or speed alteration of one vessel on the nearest approach distance of another vessel when the relative motion only of that other vessel is known, in other words that it is not necessary to determine or compute the true course and speed of the other vessel. A mathematical basis for this is set out later in this specification.
The object of the invention is to provide a navigational aid which will provide a means of determining optimum strategies in a collision avoidance situation when used in conjunction with a simple relative radar display, and at a cost which is a fraction of that of a normal sophisticated collision avoidance system as at present used in large vessels.